This invention relates to a method for regenerating an acid gas absorbing fluid effectively by allowing the acid gas absorbing fluid to absorb an acid gas and thereafter releasing the acid gas therefrom.
Conventionally, there have been proposed a variety of absorbents for removing an acid gas (e.g., CO2 or H2S) from a gaseous mixture, including an aqueous solution of an amine such as monoethanolamine or diethanolamine, and such an aqueous solution having another acid gas absorption accelerator added thereto.
Among these acid gas absorbents is methyldiethanolamine which was disclosed by F. Vidaurri at the 977th Gas Conditioning Conference, 1900. This methyldiethanolamine is advantageous in that its aqueous solution having absorbed an acid gas can release the acid gas by intermittent exposure to a reduced pressure and can hence save the thermal energy used for the regeneration of the absorbing fluid, and in that it has high acid gas-absorbing power per mole and does not corrode the equipment even if used at high concentrations. On the other hand, it has the disadvantage that its acid gas absorption rate is slow. As disclosed in Japanese Patent Laid-Open No. 198120/""94, it has been confirmed that this disadvantage can be overcome by the addition of a (lower alkyl)piperazine, resulting in a marked improvement in acid gas absorption rate. Thus, an acid gas absorbent containing methyldiethanolamine and a (lower alkyl)piperazine is known as an effective acid gas absorbent which can reduce the size of the absorption system and decrease the amount of absorbent used. Since an acid gas absorbent is being used in the form of an absorbing fluid, it will hereinafter be referred to as an absorbing fluid.
The aforesaid acid gas absorbing fluid containing methyldiethanolamine and a (lower alkyl)piperazine will show a marked increase in industrial utility if, after this absorbing fluid is allowed to absorb an acid gas, it can be regenerated by releasing the acid gas therefrom with little energy consumption and with ease, and can be reused as an absorbing fluid. However, no such method has been known in the prior art.
Consequently, the present inventors carried out various experiments and intensive investigations on the method of regenerating the aforesaid absorbing fluid, and have now completed the present invention.
Accordingly, an object of the present invention is to provide a method wherein an acid gas absorbing fluid containing methyldiethanolamine and a (lower alkyl)piperazine is allowed to absorb an acid gas and, thereafter, the acid gas absorbing fluid is regenerated by releasing the acid gas from the acid gas absorbing fluid effectively at a relatively low temperature.
That is, according to the method of the present invention, an acid gas absorbing fluid containing methyldiethanolamine and a (lower alkyl)piperazine is allowed to absorb an acid gas. Thereafter, the acid gas absorbing fluid can be regenerated by releasing the acid gas from the acid gas absorbing fluid effectively at a relatively low temperature of not less than 40xc2x0 C. and preferably 40 to 80xc2x0 C.
In the present invention, the acid gas absorbing fluid which is allowed to absorb an acid gas and subsequently regenerated contains methyldiethanolamine and a (lower alkyl)piperazine.
Specifically, methyldiethanolamine is contained in the aqueous solution as a component having a concentration of 20 to 70% by weight and preferably 40 to 60% by weight.
The (lower alkyl)piperazine is a piperazine derivative having one or two lower alkyl groups of 1 to 4 carbon atoms as a substituent or substituents on the heterocyclic ring. Of such piperazine derivatives, methylpiperazines are preferred. Among others, 2-methylpiperazine and 2,5-dimethylpiperazine are especially preferred. The (lower alkyl)piperazine is contained in the aqueous solution as a component having a concentration of 0.5 to 7.5% by weight and preferably 1.5 to 4.5% by weight.
The gases which can be treated with the acid gas absorbing fluid regenerated according to the present invention include gasified coal gas, synthesis gases, coke oven gas, petroleum gas, natural gas and the like. The gases which can be absorbed thereinto include CO2, H2S and the like.
As will be described later, the present inventors performed tests in which a simulated exhaust gas prepared by mixing nitrogen gas with CO2 gas in appropriate proportions was passed through a high-pressure CO2 separation and regeneration system having a basic construction illustrated in FIG. 1, so that an acid gas absorbing fluid was allowed to absorb CO2 under the following test conditions and, thereafter, the acid gas absorbing fluid was regenerated by releasing CO2 therefrom.
Gas flow rate: 0.12 m3N/h.
Absorbing fluid flow rate: 4 liters/h.
CO2 partial pressure in absorption tower: 10 ata.
CO2 partial pressure in regeneration tower: 1 ata.
Regeneration tower temperature: 40xc2x0 C.
Consequently, it has been found that, as shown in FIG. 7, a high regenerative effect as demonstrated by a CO2 recovery of 60% is exhibited even at a relatively low regeneration tower temperature of 40xc2x0 C. and, moreover, a very high regenerative effect is achieved at higher regeneration tower temperatures as demonstrated by a CO2 recovery of 95% or greater at 60xc2x0 C. and a CO2 recovery of 99% or greater at 80xc2x0 C.